This disclosure relates generally to rotary earth-boring drill bits. More particularly, this disclosure relates to a fixed cutter drill bit with features that improve drill bit hydraulics.
Rotary drill bits are typically mounted on the lower end of a drill string that is being rotated from the surface or by downhole motors. As the drill string is rotated, tension applied to the drill string is reduced to increase the weight on the bit so that the bit engages and drills a borehole into the earthen formation.
Two types of available drill bits are roller cone bits and fixed cutter bits. Roller cone bits often include a plurality of conical rollers that are rotatably mounted to the bit and imbedded with a plurality of cutting elements. Fixed cutter bits rely on a plurality of fixed blades angularly spaced about the bit and imbedded with a plurality of cutting elements. The cutting elements for either bit design are often formed from extremely hard materials such as polycrystalline diamond, cubic boron nitride, and tungsten carbide. The configuration or layout of the rollers, blades, and cutting elements vary widely between bit designs depending heavily on the formation to be drilled.
Both roller cone and fixed cutter drill bits utilize drilling fluid as a means to flush the drilled earth away from the drill bit and transport it to the surface. While the bit is rotated, drilling fluid is pumped through the drill string and directed out of the face of the drill bit through one or more nozzles. The drilling fluid acts to cool the bit and remove formation cuttings from the bit face and the bottom of the borehole. The behavior and performance of the drilling fluid as it acts on and around the drill bit is known as “bit hydraulics.”
In many fixed cutter drill bits, the nozzles are oriented such that high velocity drilling fluid is directed toward the formation at the bottom of the hole. As the drilling fluid impacts the formation and returns upward past the face of the drill bit, it cleans and cools the cutting elements of the drill bit. Because the drilling fluid first impacts the formation at the bottom of the hole, by the time it contacts the cutting elements of the drill bit the velocity of the drilling fluid is reduced from its initial velocity upon exiting the nozzles. This reduced velocity may limit the amount of cooling and cleaning that can be achieved by the drilling fluid.
Inadequate cooling of the cutting elements may lead to decreased life of the drill bit due to high temperatures supporting increased erosion and wear of the cutting elements. If formation cuttings are not cleaned from the cutting elements of a fixed cutter drill bit, formation materials may build up on the cutting elements and greatly increase drilling times.
Some types of earthen formations, sometimes referred to as plastic shales or simply plastic formations, are difficult to drill efficiently. Upon action of the cutting elements, these formations generate cuttings in the form of long ribbons of drilled material that tend to accumulate between the cutting elements of the drill bit, a phenomenon called “bit balling.” When the bit is balled, it is no longer possible for the cutting elements to engage the formation, regardless of the weight applied on the drill bit. Drilling of the borehole pauses for the drill bit to be retrieved and cleaned by means other than pumping drilling fluid through the drill string.
Thus, there is a continuing need in the art for methods and apparatus for improved fixed cutter drill bit designs that improve drill bit hydraulics.